This invention relates to the use of photodiodes, and particularly to the interconnecting of a photodiode within an electronic circuit for improved efficiency of operation.
Photodiodes are known electronic components which generate electrical current in response to radiant energy. Principal uses of photodiodes are in photodetectors such as image sensors and photospectrometers.
Typically, a plurality of individual photodiodes are disposed in side by side array to define a light receiving area. A light pattern to be analyzed is projected onto the light receiving area and each photodiode generates a current in proportion to the intensity of the light falling thereon. If the original source of light has been separated by wavelength, so that each photodiode receives light of a different wavelength, the photodiode array functions as a photospectrometer for detecting and measuring the different wavelength components of the light source.
The output of each photodiode of the array is electrical current (referred to as "photocurrent"). Ideally, all the photodiodes forming a photodetector should have identical characteristics whereby the relative amplitudes of the current outputs of the various photodiodes precisely reflect the relative intensities of light incident thereon. Some variations in characteristics of the photodiodes are inevitable, however, and a task is to minimize the effects thereof.
In certain photodetector applications, described hereinafter, the photodiodes are operated at zero voltage bias and, when so operated, an equivalent circuit of the photodiodes comprises a radiant energy responsive current generator having two output terminals. A shunt resistor is connected between the two output terminals in parallel with the current generator, and a resistor is connected in series between the generator and one of the terminals. As described hereinafter, the shunt resistor represents a path for leakage current within the photodiode, and the series resistor represents intrinsic resistance to current flow outwardly from the photodiode.
The two resistors form a current divider internally of the photodiode in the path of the current from the current generator. That current which flows through the series resistor and to the one output terminal is available to contribute to the output signal of the photodiode. The current which flows through the shunt resistor circulates within the photodiode and is not available as output current.
Clearly, for maximum signal output, the shunt resistance is preferably as high as possible and the series resistance is as low as possible.
The magnitudes of the two photodiode resistors are a function of the design and fabrication of the photodiodes. The present invention is not concerned with the fabrication of photodiodes, but with the arrangements used for connecting photodiodes to the input nodes of amplifier circuits. The impact of such connecting arrangements is as follows.
With any given photodiode including the aforedescribed internal current divider, it is recognized that the functioning of the current divider can be influenced by how the photodiode is used. Specifically, if the path for current from the one output terminal from the photodiode to the input node of the amplifier circuit includes a resistance, such resistance can add to the series resistance portion of the photodiode current divider. Thus, a greater proportion of the current generated by the photodiode is lost to internal circulation.
Based upon the foregoing, it has been accepted heretofore that the output terminals of the photodiodes should be connected to the input nodes of respective amplifier circuits by connecting paths having the lowest possible resistances and, preferably, of identical resistance values. While known connecting arrangements largely accomplish this, some resistance is inevitably present in the connecting paths and, depending upon the geometries of the photodetecting devices, the resistances can vary from path to path. Thus, not only do the path resistances tend to reduce the output currents of the photodiodes, but the variable path resistances variably influence the photodiodes for magnifying existing differences among the photodiodes.
The present invention reduces the adverse effects of connecting path resistances.